A general approach to achieve a higher density circuit board is to fabricate a multi-layer circuit board by stacking circuit boards in multiple layers. The multi-layer circuit board is generally fabricated by the steps of {circle around (1)} stacking an electrically insulating layer (2) on an internal layer board having formed thereon an electrically insulating layer (1) and a conductor circuit (1), that is formed on the surface of the electrically insulating layer, on a conductor circuit formation surface of the internal layer board, {circle around (2)} forming a conductor layer (2) on the electrically insulating layer (2) and, {circle around (3)} if necessary, stacking the electrically insulating layers and the conductor circuits in several stages. Further, this method includes the step of {circle around (4)} forming through-holes and via-holes and embedding an electrically conductive material in these holes to electrically connect the conductor circuits, inside the multi-layer circuit board, with one another.
If foreign matter such as dust and dirt exist in the production process of the multi-layer circuit board, they impede the formation of the through-holes and the via-holes, and it often becomes impossible to stack the electrically insulating layers and the conductor circuits at a high density. Therefore, the production process of the multi-layer circuit board has been carried out, in the past, inside a clean room, for example, lest dust and dirt adhere to the surface of the multi-layer circuit board.
A technology for achieving a higher density has been required for recent multi-layer circuit boards so as to allow reduction of size and weight and higher functions of electronic devices used with the multi-layer circuit boards. At present, the thickness of the electrically insulating layer of not greater than 0.1 mm per layer and a wiring density of not greater than 0.05/0.05 mm in connection with wiring width/wiring gap have been required. To accomplish this object, it is necessary to achieve a process for miniaturizing wiring at a higher level of accuracy and to provide laminate layers having higher flatness and higher adhesion.
In the production of multi-layer circuit boards, however, removal of foreign matters having a particle size of 30 to 50 μm or below has scarcely been taken into consideration. It has been known that when an insulating material produced under such an environment is employed, the formation of wiring having a miniature structure or the formation of the through-holes and the via-holes becomes more difficult as the wiring gap and the diameter of the through-holes or the vial-holes become smaller, an electrically conductive material cannot completely be filled into the through-holes or the via-holes, and stacking of layers with high adhesion and high flatness becomes difficult. Even when the stacking operation of the insulating films is carried out in a clean environment in which a level of particles floating in air is managed, conduction defect in minute circuit patterns or undesirable short-circuits, that may presumably result from the admixture of the foreign matter, occurs.
On the other hand, a solution casting method that applies a solution of a curable resin composition containing a curable resin to a support and then removes a solvent is generally known as a method of forming the electrically insulating material.
It is also known that various factors such as an evaporation speed of the solvent, an ambient temperature and humidity, a concentration of the resin solution and the coating thickness remarkably affect the properties of the film obtained by this solution casting method. Therefore, attempts have been made in the past to variously regulate these factors in order to obtain films having excellent properties.
For example, (1) Japanese Unexamined Patent Publication (Kokai) No. 3-316380 proposes a method that arranges a planar heat generating member below a support inside a room temperature drying zone extending from a coating portion to a drying zone inside an oven, and supplies heat from the planar heat generating member. In a production method of a film involving the steps of applying a polymer material to a support at a coating portion, conveying the support to a drying zone spaced apart by a predetermined distance from the coating portion, and evaporating a solvent inside the drying zone to form a film, (2) Japanese Unexamined Patent Publication (Kokai) No. 9-70841 describes a production method of a film that arranges at least one temperature sensor for a non-dried coating film on the support and forms the film while the temperature of the non-dried coating film is measured. These methods are directed to solve the problem that latent heat of evaporation of the solvent after coating but before drying of the solvent deprives the non-dried coating film applied, as well as the support, of heat and invites a drastic drop of the temperature with the result that the moisture in the room temperature atmosphere dews on the surface of the non-dried coating film and the surface of the coating film changes to a condition like that of ground glass, thereby deteriorating the transparency of the film. In a solution casting method for producing a film from a polymer solution by casting a polymer solution dissolved in an organic solvent to a support consisting of a traveling endless belt or a drum surface to form a film, and then drying the film, (3) Japanese Unexamined Patent Publication (Kokai) No. 11-58425 conducts first stage drying by blowing, in the drying step, drying air at a velocity of 0.1 to 2.0 m/sec onto the film formed on the support by casting until the temperature reaches a gelling temperature so set as to correspond to the components of the polymer solution, and then conducts second stage drying for more quickly drying the film after the film reaches the temperature of completion of gelling. This solution casting method for forming the film divides the drying step into two steps and regulates the velocity of drying air for each drying step so as to suppress an adverse influence on flatness of the film due to variance of drying of the film and the drop of quality, and to improve productivity.
Among the known methods described above, the methods (1) and (2) do not take the drying condition, such as the drying temperature and the drying time inside the drying zone, into consideration, and are not sufficient to obtain an insulating film having superior flatness. The method (3) divides the drying step into two steps, pays a specific attention to management of the velocity of the drying air in the drying step and can suppress deterioration of flatness of the film due to variance of drying of the film. However, this method does not sufficiently consider the drying temperature and the drying time in the drying step, and its effect is not entirely sufficient. Furthermore, this method cannot easily provide an insulating material having a low content of foreign matter and bubbles.
In view of the problems described above, this invention aims at providing an insulating material with a low content of foreign matter and bubbles, that reduces the number of insulation defects and conduction defects, and is suitable for providing a multi-layer circuit board excellent in flatness, and a production method for such an insulating material.